Anaerobiosis of Pseudomonas aeruginosa

Journal of Bacteriology and Virology 2010. Vol. 40, No. 2 p.59 – 66 DOI 10.4167/jbv.2010.40.2.59

Review Article

Anaerobiosis of Pseudomonas aeruginosa: Implications for Treatments of Airway Infection *

Sang Sun Yoon

Department of Microbiology and Brain Korea 21 Project for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea Pseudomonas aeruginosa, as an opportunistic pathogen, establishes a chronic infection in the respiratory track of patients suffering from pneumonia and bronchiectasis, including cystic fibrosis. Biofilm formation inside the oversecreted mucus layer lining the patient airway and production of virulence factors, a process controlled by quorum sensing, are considered to be the major virulence determinants in P. aeruginosa pathogenesis. Recently, an abnormally thickened mucus layer was proven to be anaerobic. Given the fact that currently used antibiotics are less effective under anaerobic environments, these new findings lead us to change the way we confront P. aeruginosa infection. This article reviews pathological features of patient airways that become susceptible to P. aeruginosa infection and bacterial adaptation that contributes to the prolonged survival inside the patient airway. Key Words: Pseudomonas aeruginosa, Anaerobic environments, Biofilm, Quorum sensing

organized communities called biofilms and has been

I. Pseudomonas aeruginosa

served as a model organism to explore bacterial biofilm formation (5). Biofilm is defined as a microbial "living"

P. aeruginosa has long been considered to be a classic

biomass grown on an aggregate or on a surface with distinct

example of an opportunistic pathogen (1). The organism

architecture (6, 7). Compared to its free living counterpart

does not normally cause infections in individuals with

(i.e. planktonic cells), bacteria grown as biofilm are

intact immune systems, but immunocompromised patients

refractory to a variety of antimicrobial reagents including

are particularly at risk for P. aeruginosa infection.

H2O2 (8), a range of antibiotics (9, 10), and various heavy

P. aeruginosa, a gram-negative bacterium, is remarkably

metals (11). Moreover, bacterial biofilm is more resistant to

versatile in terms of the metabolism, and thus, can maximize

host immune clearance (12).

its survival fitness in various environments including human

P. aeruginosa has been notorious for its high level

hosts (2). The organism, however, is strictly dependent on

antibiotic-resistance, arguably one of the most important

respiration to generate energy and is often classified as a

virulence features of clinically isolated P. aeruginosa.

non-fermenting bacterium (3, 4).

Recently, we reported that over 76% of the Korean

In nature, this gram-negative bacterium is found in highly

pneumonia patients isolates showed resistance to more than one antimicrobial agent, currently employed to combat P.

Received: June 8, 2010/ Revised: June 10, 2010 Accepted: June 11, 2010 * Corresponding author: Sang Sun Yoon, Ph.D. Department of Microbiology, College of Medicine, Yonsei University, 250 Seongsanno, Seodaemun-gu, Seoul 120-752, Korea. Phone: +82-2-2228-1824, Fax: +82-2-392-7088 e-mail: [email protected]

aeruginosa infection (Yoon et al., in press). Mechanisms by which P. aeruginosa acquires antibiotic-resistance have been extensively studied and reviewed in detail elsewhere (13~16). 59

60

SS Yoon

Bacterial virulence factors are (i) molecules produced by

A Normal

Mucus layer

microbial pathogens that induce specific disease symptoms in the host and (ii) mechanisms by which pathogens deliver (or secrete) those molecules. But, in broad terms, virulence factors include any factors that contribute to the successful colonization of host tissues. As an extracellular pathogen, P. aeruginosa secretes an array of virulence factors, whose

Airway epithelium Goblet cell

Periciliary liquid layer

production is controlled by quorum sensing, a cell-density dependent gene regulatory pathway. Effectors to be secreted include elastase (17, 18), alkaline protease (19, 20), exotoxins (21, 22), phospholipase (23, 24), and pyocyanin

B Disease Thickened viscous mucus Non-functional periciliary liquid layer

(25). These molecules exert toxic effects on human hosts by directly degrading host tissues or eliciting oxidative stress.

II. Abnormal mucus environments in airway diseases Under normal airway environments, invading microorganisms are usually expelled and/or cleared by the upper airway innate immune defense system that includes the mucociliary clearance (26~28). P. aeruginosa being an opportunistic pathogen, however, can cause persistent

Figure 1. Schematic comparison between normal (A) and diseased (B) airway mucus environments. Maintenance of periciliary liquid layer (PLL) with constant depth and appropriate movement of the mucus layer on top of the PLL, which mediates the mucociliary clearance, is achieved in normal airways. In diseased states, however, PLL is depleted and an abnormally oversecreted (and thus, highly viscous) mucus layer is formed. This mucus layer is highly susceptible to bacterial colonization.

infection in patients with abnormal airway mucus secretion. Patients suffering from cystic fibrosis (CF) (1, 29),

resulting in the dehydration of PLL and thus the formation

bronchiectasis (30, 31), and pneumonia (32) are especially

of a stagnant mucus layer (1). Bronchiectasis (BE) is a

vulnerable to P. aeruginosa infection. Among many

disease state where the bronchial tree is irreversibly dilated.

pathological symptoms, aforementioned airway diseases

BE is caused by early childhood bacterial infections or

are characterized with the noticeable oversecretion of

pulmonary tuberculosis and patients with BE are highly

mucus on top of the airway epithelium that debilitates the

susceptible to secondary infection by microbial pathogens

mucociliary clearance activity (1, 33). As depicted in

including P. aeruginosa. BE is also featured with mucus

Figure 1, mucus hypersecretion is often accompanied with

hypersecretion and impaired mucociliary clearance activity

the depletion of the periciliary liquid layer (PLL) and

(33). A recent report demonstrated that neutrophil protease

subsequent loss of mucociliary clearance activity.

present in large quantity in sputum samples from the BE

CF is a genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) -

patients stimulates the secretory response in tracheal submucosal glands (33).

gene coding for Cl transport channel across the apical

Much evidence indicated that the oversecreted and

surface of secretory cells (34). In CF, hyperactivation of

stationary mucus layer provides a nice "habitat" for P.

+

epithelial Na channel (ENaC), an event that occurs due to

aeruginosa to colonize and proliferate (1, 36). Importantly,

the mutation in the CFTR gene (35), drives the isotonic

this abnormally altered mucus layer (Fig. 1B) also renders

absorption of H2O and ions into the airway epithelium

the host immune system ineffective against P. aeruginosa

Anaerobiosis of Pseudomonas aeruginosa : Implications for Treatments of Airway Infection

61

A Aerobic respiration 2H+

Cyt c

H+ e¯

NADH DH

2H+

e¯

e¯

H2O

Cyt

Cyt

bc1

oxidase

e¯ NADH

Cytoplasm

O2 2H

+

H+

2H+

B Anaerobic respiration

e¯

NIR

H+

2H+

e¯

NADH DH

NO2¯

NO

NAR

N2O

NOR

Cyt c

e¯

N2OR

H+

e¯

N2

Periplasm Cyt bc1

e¯

Cytoplasm

e¯ NADH

Periplasm

e¯

NO3¯ 2H+

Denitrification: NO3¯

H+

NAR

H

NO2¯

NO2¯

NIR

NO

NOR

N2O

+

N2OR

N2

Figure 2. Aerobic (A) vs. anaerobic (B) respiratory pathways in P. aeruginosa. P. aeruginosa can use either oxygen or nitrate/nitrite as electron acceptors in the electron transport chain. NADH DH, NADH dehydrogenase; Cyt bc1, cytochrome bc1 complex; Cyt oxidase, cytochrome oxidase; NAR, nitrate (NO3-) reductase; NIR, nitrite (NO2-) reductase; NOR, nitric oxide reductase; N2OR, nitrous oxide (N2O) reductase.

infection. Despite a vigorous and rapid influx of neutrophils into the infected airways (37), accompanied by production

III. Anaerobic growth of P. aeruginosa

of high titers of specific antibodies (38), P. aeruginosa infection persists and lung function progressively declines.

Being an obligate respirer, P. aeruginosa is also capable

Recently, the stagnant mucus layer lining the airway of

of generating energy even in the absence of oxygen using

chronic CF patients was reported to be anaerobic (3, 36).

NO3- (nitrate) or NO2- (nitrite) as alternative electron

The lack of oxygen potential is ascribed to (i) the limited

acceptors (1, 4). The P. aeruginosa genome harbors clusters

oxygen transport into the mucus layer due to its increased

of genes encoding enzymes for anaerobic respiration.

viscosity and (ii) a high rate of oxygen consumption by

Figure 2 compares the electron transport pathway between

immune-related and airway epithelial cells. This new

aerobic and anaerobic growth conditions. NADH, produced

observation provides a new insight into the establishment

by the glycolysis and TCA cycle, feeds an electron to the

of P. aeruginosa infection under anaerobic condition.

inner-membrane bound NADH dehydrogenase (39) to initiate the electron transport pathway. During the sequential electron transports to cytochrome bc1 complex (40) and

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SS Yoon

cytochrome oxidase (41), H+ ions are pumped out across +

resistance to a variety of antimicrobial treatments. Molecular

the inner membrane generating pH gradient. Then, H ions

basis that accounts for such a high-level resistance has

re-enter the cytoplasm via ATP synthase (42) to produce

been extensively studied. Recently, a role of periplasmic

ATP.

glucan encoded by the ndvB gene has been proposed to

As shown in Figure 2B, pH gradient across the inner NO3-

NO2-

explain the antibiotic resistance of P. aeruginosa biofilm

is

(49, 50). While a mutant defective in ndvB can form biofilms

supplemented even under the anaerobic condition. Anaerobic

with normal structural features, the mutant exhibited

respiration, often called denitrification (4) involves four

enhanced sensitivity to tobramycin, an aminoglycoside-

membrane can still be generated when

NO3-

or

to N2. Each step is mediated by

type antibiotic. It was also found that the mutant strains

respiratory enzymes; nitrate reductase (NAR), nitrite

showed decreased binding to tobramycin, suggesting that

reductase (NIR), nitric oxide reductase (NOR), and nitrous

periplasmic glucan may provide a physical barrier to prevent

oxide reductase (N2OR). It is of interest to note that NO, a

tobramycin from penetrating into the cytoplasm.

reduction steps from

toxic chemical to microorganisms (4), is produced as a

Recently, it was revealed that P. aeruginosa forms more

byproduct during the denitrification process. This is

robust biofilm during anaerobic respiration than they do

analogous to the unavoidable production of reactive oxygen

when they respire aerobically (3). Since oxygen transfer to

species in aerobically respiring cells. P. aeruginosa, however,

the depth of biofilm can be significantly limited (51), it has

can minimize the accumulation of the toxic NO during the

been postulated that "anaerobic" local regions may exist

anaerobic growth by the activity of NOR (4).

within mature biofilms. This result, however, shows that

NO3-

NO2-

were detected in larger

P. aeruginosa is actively responding to the anaerobic

quantity in sputum or exhaled condensate of patients with

respiration in order to form more robust biofilm, a resistant

pulmonary exacerbation of CF than those obtained from

mode of growth. This result further suggests that P.

normal individuals (43, 44). This result suggests that P.

aeruginosa airway infection is clearly associated with the

aeruginosa proliferates well inside the anaerobic mucus

biofilm formation under anaerobic conditions. Understanding

layer exploiting the compounds produced by the host and

the molecular basis behind this anaerobiosis-induced robust

may provide an insight into why P. aeruginosa has been

biofilm formation will provide better insight into the P.

such a competitive colonizer in the patient airways.

aeruginosa pathogenic mechanisms leading us to come up

Importantly,

and

with novel strategies to treat the infection.

IV. P. aeruginosa biofilm and a new emerged concept on the enhanced biofilm formation during anaerobic respiration

V. P. aeruginosa quorum sensing and the future direction

Biofilm formation is often described as a process by

P. aeruginosa fine-tunes its virulence by a process of

which bacterial cells develop into a sessile community (45).

inter-cellular communication known as quorum sensing

Steps that can be clearly distinguished during this process

(QS). In QS, P. aeruginosa produces, secretes, and responds

include (i) initial attachment of free living bacteria to

to extracellular signal molecules, called autoinducers, to

abiotic or biotic surface (46), (ii) microcolony formation

regulate the expression of genes involved in biofilm

with ensuing cell division (47), (iii) secretion of matrix

formation (52) and production of diverse virulence factors

molecules and growth of microcolonies into macrocolony

including exotoxin A (53), elastase (54), alkaline protease

(48), and (iv) differentiation into mature biofilm with

(55), rhamnolipid (54), and pyocyanin (25). Expression of

3-dimensional architecture (5).

genes encoding superoxide dismutase and catalase, which

Biofilm formation has been a major problem due to its

mediate oxidative stress responses, is also controlled by QS


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(56). The role of QS in P. aeruginosa virulence was clearly

formation supporting the presence of a novel function of P.

demonstrated in studies using infection models with a

aeruginosa QS. Further investigation is warranted to better

range of different living hosts (57~59) and cultured host

understand the QS operation during anaerobic growth, a

cells (60, 61).

mode of proliferation that occurs in the patient airway.

There are three well-characterized QS systems in P. aeruginosa: las, rhl, and pqs. The las and rhl systems were

VI. Conclusions

initially identified to be essential for elastase and rhamnolipid production, respectively (62, 63). Each system

Although patient airways are equally exposed to diverse

is composed of its own transcriptional activator protein

bacterial pathogens, P. aeruginosa has been a major

(LasR or RhlR) and cognate autoinducer synthase, LasI or

microorganism that successfully colonizes and establishes

RhlI, that produces N-(3-oxododecanoyl)-L-homoserine

persistent infection in the airway. P. aeruginosa airway

lactone and N-butyryl-L-homoserine, respectively. Each

infection should now be approached as an anaerobic disease

autoinducer molecule binds to its cognate transcriptional

of lung and this new idea necessitates further research

activator, LasR or RhlR, and this complex then apparently

directed on identifying new targets, inhibition of which will

binds to RNA polymerase, which results in transcriptional

decrease bacterial virulence or survival under anaerobic

activation of QS regulated genes.

condition. Because biofilm and QS are two major arms of

Another arm of P. aeruginosa QS is a system where the

virulence mechanisms of this clinically important pathogen,

DNA-binding affinity of MvfR (PqsR), an important

future therapeutic strategies for the treatment of airway

virulence-associated transcriptional regulator, is enhanced

infection should include molecular-level understanding of

upon binding with pseudomonas quinolone signal (PQS)

anaerobiosis-induced biofilm and QS.

(64, 65). Mounting evidence indicated that PQS is also a major player in the complex intertwined P. aeruginosa QS

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